That's not an excuse! He is right to claim foul Play here. If deposits were suspended on 10th of may, all deposit option for that coin is suppose to be removed effectively and immediately disabled for users.
Mr okorieemmanuel, How sure are you that the deposit option was still active?
What was the point of gate.io announcing suspension of deposit if they still left the deposit option active?
Is OP the only person who fell victim? Why are we not seeing more of such complaints about the "foul play" on Twitter, their Reddit or Trustpilot?
Bitcoin_Arena is correct when he says that an announcement was made that deposits and withdrawals will be suspended 4 days before OP sent those coins. But as okorieemmanuel rightly said, users of Gate.io shouldn't have to go search on Twitter or Gate.io articles whether or not they can deposit, withdraw, or trade their assets. Such instructions need to be available on the wallet page, which shouldn't allow you to display a receiving address at all due to the suspension in trading.
When it comes to accusations of the exchange doing arbitrage trading, that has to be proven, and I doubt OP can do that.
it's also possible that OP just made a deposit of Luna into the exchange using his Luna deposit address that he already has saved somewhere.
For example, I have my Binance or Bybit deposit addresses saved in my noncustodial wallet. I can send coins to my account minus checking my deposit option via the exchange, which is a very bad practice in case they have disabled deposits, and I am not aware.